The growing number of decentralized power generation systems using renewable energy sources like, for instance, photovoltaic energy, wind energy or biomass energy lead to high demand of switching mode power supply systems for converting DC-power into AC-power useable for feeding into the utility grid. For that, energy conversion DC/AC-converters—with or without an upstream connected DC/DC—converter are used. A power loss during the energy conversion has to be as low as possible. On the other hand, DC/AC-converters connected to the utility grid have to comply with a maximum allowed total harmonic distortion (THD) when feeding the AC power into the utility grid. Another requirement apart from the high efficiency for the grid tied DC/AC-converters is a large power density. A power supply system has to be able to convert a large power amount with a mass—and equivalently a cost—of the power converter as low as possible.
In order to react on these requirements, todays power supply systems are using multiple switching paths connected in parallel to each other. With each switching path the nominal power to be converted by a particular power supply system can be increased without having a tremendous effect on its mass or the cost—at least compared to the case in which two power supply systems are connected in parallel. In order to minimize a voltage ripple—either at a DC-link capacitance of a DC/AC-converter or at an output capacitance of a DC/DC converter—the multiple switching paths of the power supply system are operated in an interleaved manner.
However, that interleaved control manner typically leads to additional power losses due to a generation of circulating currents which flow from one switching path to another parallel connected switching path without leaving the output of the power supply system.
The document WO 2014/194933 A1 discloses a five-level active neutral-point clamping inverter for converting a bipolar DC voltage to a three-phase AC output voltage. The converter comprises first, second and third input terminals (P, MP, N) and first, second and third output terminals. The inverter further comprises first, second and third multistate switching cells (MSSC), each comprising three input terminals respectively connected to the input terminals of the inverter and respectively first, second and third output terminals. The output terminals of the first, second and third multi-state switching cell are connected via an inductor to said first, second and third output terminal of the inverter. Furthermore each respective output terminal of the inverter is connected to said second input terminal (MP) of the inverter via a respective capacitor (Ca, Cb, Ce). Each one of the multi-state switching cells (MSSC) comprises a separate autotransformer, wherein each separate autotransformer comprises end terminals and an intermediate terminal.
The separate autotransformers used in the multistate switching cell, however, require a relatively large installation space. Additionally, separate autotransformers comprise a relatively large mass and therefore are expensive components.
The document US 2008/094159 A1 as well as its corresponding patent family member EP 1 914 868 A1 disclose a three-phase AC or two-phase DC choke arrangement of a frequency converter, comprising: a magnetic core with a plurality of phase specific pillars having phase specific windings wound there-around or a plurality of branch specific pillars having branch specific windings wound there-around. The phase-specific windings of the AC choke arrangement or the branch-specific windings of the DC choke arrangement are adapted to filter differential mode currents. The choke arrangement comprises an additional pillar without the phase-specific or branch-specific windings fitted around it and arranged in the magnetic core for damping common-mode currents. The common mode currents are damped by means of a common-mode impedance formed by the additional pillar and the windings arranged around the phase-specific or branch-specific pillars.
The article “Integrated Inductor for Interleaved Operation of Two Parallel Three-phase Voltage Source Converters” by G. Gohil et al. discloses an inductor assembly for two interleaved Voltage Source Converters. The disclosed inductor assembly is able to combine both line filter and circulating currents filter functionality within one magnetic structure. The magnetic core is composed of three phase legs, a common leg and three bridge legs between the phase legs. Each phase leg comprises two inductor coils each one corresponding to a different one of the two voltage source converters. A high permeability material is used for the phase legs and the common leg, whereas the bridge legs are realized using a laminated iron core. An air gap has been inserted in each of the bridge legs.
The construction of the inductor assembly is relatively complex and comprises different magnetic materials within the core. The total mass of the inductor assembly as well as its respective installation space is still relatively large. This might cause disadvantageous impacts with regards to manufacturability and material costs.